Bulldozers and other heavy-duty vehicles are frequently required to operate both forward and backward with a plurality of speed ratios or "gears," generally a low speed ratio or "first gear," an intermediate speed ratio or "second gear" and a high speed ratio or "third gear." In order to transmit the requisite torque from an internal-combustion engine to an output shaft, designed to drive the traction wheels of the vehicle and possibly other loads as well, use is often made of constant-mesh transmissions comprising a gear train wherein gears on parallel shafts are in permanent engagement with each other but may be selectively coupled with or decoupled from the respective shafts by clutch gears slidable on splined shaft portions under the control of a fluidic actuator such as a pneumatic or hydraulic jack. In changing from a lower speed ratio to a higher one, a procedure known as upshifting, the engine-driven input shaft of the transmission must be accelerated with reference to the output shaft for a smooth transition. Conversely, a change from a higher speed ratio to a lower one--known as downshift--requires a relative acceleration of the input shaft. With a manual transmission, therefore, the driver is expected to depress the acceleration pedal in the brief interval during which the shafts are effectively decoupled on the upshift and to throttle the engine during the corresponding interval on the downshift.
The use of speed sensors for an automatic upshifting and downshifting, in response to variations in shaft speed, is also known in the art. With such automatic transmissions the driver can no longer change the engine speed at just the right moment, yet this speedup or slowdown is not required with transmission systems of the planetary-gear type using hydraulically operated clutches and brakes without gear teeth.